Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus and method include an illumination system that supplies a beam of radiation, an array of individually controllable elements that pattern the beam, and a projection system that directs the patterned beam a substrate supported on a substrate table. The projection system defines a pupil. Either the pupil or the array of individually controllable elements is imaged onto a target portion of the substrate. The projection system includes an array of lenses with each lens in the array arranged to direct a respective part of the patterned beam onto a respective part of the target portion of the substrate. In one example, each of the individually controllable elements is selectively controllable to direct a respective part of the beam away from the pupil such that the proportion of the beam passing through the pupil is adjusted. In one example, the individually controllable elements are arranged in groups, such that radiation deflected by each element in one group is directed towards the same lens in the lens array. In one example, the individually controllable elements in any one group are controlled together to direct radiation in different directions away from the pupil, such that the pattern imparted to the beam by that group of elements is substantially symmetrical with respect to the pupil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/899,302, filed Jul. 27, 2004 (U.S. Pat. No. 7,142,286), which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. The lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs), flatpanel displays, and other devices involving fine structures. In aconventional lithographic apparatus, a patterning means, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern corresponding to an individual layer of theIC (or other device), and this pattern can be imaged onto a targetportion (e.g., comprising part of one or several dies) on a substrate(e.g., a silicon wafer or glass plate) that has a layer ofradiation-sensitive material (e.g., resist). Instead of a mask, thepatterning means may comprise an array of individually controllableelements that generate the circuit pattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion in one go, and scanners, inwhich each target portion is irradiated by scanning the pattern throughthe beam in a given direction (the “scanning” direction), whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

The conventional lithographic apparatus delivers a beam to the substratethrough a lens assembly in which each of the lenses is arranged inseries along the beam projection direction. The lens component closestto the substrate is a single lens through which all of the beam passes.

An alternative design approach uses a series of lenses arranged alongthe beam path, but the lens component closest to the substrate is in theform of a two dimensional array of small lenses. Each of the smalllenses focuses a respective part of the beam onto a respective part ofthe substrate. Lithography systems using this design are generallyreferred to as microlens array imaging systems or MLA systems.

In the lithographic apparatus incorporating the MLA systems, it ispossible to rely upon an array of individually controllable elementsthat provide a black or white effect. An individual element eitherreflects a beam directly towards an individual lens of the MLA (“white”)or directs light away from the lens of the MLA array (“black”). Theeffect is the same as turning ON or turning OFF a beam componentdirected towards that single lens, which delivers either a maximumintensity beam or a zero intensity beam.

It is desirable have a gray-tone capability, which is the capability ofdelivering to the substrate a light intensity intermediate a maximum anda zero intensity. A gray-tone capability is desirable in lithographicapparatus relying upon MLA arrays. It has been proposed to providegray-tone capability by progressively adjusting the position ofindividual mirrors in a mirror array to progressively reflect light awayfrom the center of a respective lens in the lens array. A single beam oflight from a single mirror of a mirror array is progressively displacedrelative to a respective lens of the lens array.

In a lithographic apparatus in which a single mirror is used to reflectlight to a single lens of the array and that mirror is progressivelydisplaced to direct the reflected beam progressively away from thatsingle lens, not only the intensity of the beam reaching the substrateis modulated. For example, in MLA systems in which the pupil of theprojection optics is imaged on the substrate, the pupil is asymmetricalwith respect to the beam of light which illuminates a single lens of thearray. Thus, both the intensity and position of the spot of illuminationat the substrate are modulated as a result of deflection of the mirrorelement. In MLA systems in which the displaceable mirror elements areimaged on the substrate, the beam reaching the substrate from anindividual mirror will not be telecentric, The non-telecentricity willvary with displacement of the mirror, resulting in the position of thespot of illumination at the substrate varying with focus.

Therefore, what is needed is an improved lithographic apparatus anddevice manufacturing method that can be used in a microlens arrayimaging system to provide gray-tone capability.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, there is provided alithographic apparatus comprising an illumination system for supplying abeam of radiation, an array of individually controllable elementsserving to impart to the beam a pattern in its cross-section, asubstrate table for supporting a substrate, and a projection system forprojecting the patterned beam onto a target portion of the substrate.The projection system defines a pupil and comprises an array of lenses.Each lens in the array is arranged to direct a respective part of thepatterned beam towards a respective part of the target portion of thesubstrate.

In one example, each of the individually controllable elements isselectively controllable to progressively direct a respective part ofthe beam away from the pupil, such that the proportion of the beampassing through the pupil is progressively adjusted.

In one example, the individually controllable elements are arranged ingroups, such that radiation is directed by each element in one grouptowards the same lens in the lens array. The individually controllableelements in any one group are controlled together to progressivelydirect radiation in different directions away from the pupil, such thatthe pattern imparted to the beam by that group of elements issubstantially symmetrical with respect to the pupil.

In one example, two or more mirrors, or other progressively adjustablepattern imparting devices, are used to illuminate a single lens of thearray and those mirrors, or other pattern imparting devices, arecontrolled to create a symmetrical arrangement in which the illuminationspot position is decoupled from the illumination spot intensity.

In one example, two individually controllable elements, such as mirrorelements, may make up a single group. Each of the two elements areprogressively adjusted, so as to direct radiation away from the pupil indirections inclined at 180° to each other.

In one example, three elements progressively direct radiation away fromthe pupil in directions inclined at 120° intervals to each other.

In one example, four elements progressively direct radiation away fromthe pupil in directions inclined at 90° intervals to each other.

In these above examples, the individually controllable elements can bemirrors, each of which can be tilted progressively away from a positionin which radiation reflected by that mirror is symmetrical with respectto the pupil. A beam splitter can be used to reflect radiation from theillumination system to the individual controllable element and totransmit light from the individually controllable element towards thearray of lenses. The pupil can be defined by a projection lens contrastaperture plate.

Another embodiment of the present invention provides a devicemanufacturing method comprising the steps of providing a substrate,providing a beam of radiation using an illumination system, using anarray of individually controllable elements to impart to the beam apattern in its cross section, projecting the patterned beam of radiationonto a target portion of the substrate through an array of lenses eachof which is arranged to direct a respective part of the patterned beamtowards a respective part of the target portion, and selectivelycontrolling each of the individually controllable elements progressivelyto direct a respective part of the beam away from the pupil such thatthe proportion of the beam passing through the pupil is progressivelyadjusted. The individually controllable elements are controlled ingroups, such that the elements in any one group direct radiation towardsthe same lens in the lens array and are controlled progressively todirect radiation away from the pupil in different directions such thatthe pattern imparted to the beam by that group of elements issubstantially symmetrical with respect to the pupil.

In one example, the present invention described in the embodiments andexamples above is used in a lithographic apparatus relying upon arraysof individually controllable elements to impart a pattern to theprojected beam.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 depicts features of a lithographic apparatus of a type to whichone or more embodiments of the present invention may be applied.

FIG. 2 is a simplified illustration of an optical projection systemincorporating a microlens array, according to one embodiment of thepresent invention.

FIG. 3 is a simplified illustration of components of the system shown inFIG. 2 and includes a displaceable substrate table, according to oneembodiment of the present invention.

FIG. 4 is a schematic representation of the orientation of spots oflight projected onto a substrate in the system illustrated in FIG. 3,according to one embodiment of the present invention.

FIG. 5 is a schematic representation of a contrast device and contrastaperture in a first disposition of the contrast device, according to oneembodiment of the present invention.

FIG. 6 corresponds to FIG. 5 after a change in the disposition of thecontrast device, according to one embodiment of the present invention.

FIGS. 7 and 8 illustrate the distribution of a radiation beam relativeto the contrast aperture given the dispositions of the contrast deviceshown in FIGS. 5 and 6, according to embodiments of the presentinvention.

FIG. 9 shows an arrangement in which two contrast devices are adjustedto produce a radiation distribution that is symmetrical relative to acontrast aperture, according to one embodiment of the present invention.

FIG. 10 illustrates a distribution of radiation given the dispositionsof the two contrast devices shown in FIG. 9, according to one embodimentof the present invention.

FIGS. 11 and 12 illustrate distribution of radiation in an arrangementwith three symmetrically disposed contrast devices and foursymmetrically disposed contrast devices, respectively, according tovarious embodiments of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers mayindicate identical or functionally similar elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS Overview and Terminology

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of integrated circuits (ICs),it should be understood that the lithographic apparatus described hereinmay have other applications, such as the manufacture of integratedoptical systems, guidance and detection patterns for magnetic domainmemories, flat panel displays, thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion,” respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (e.g., atool that typically applies a layer of resist to a substrate anddevelops the exposed resist) or a metrology or inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The term “array of individually controllable elements” as here employedshould be broadly interpreted as referring to any device that can beused to endow an incoming radiation beam with a patterned cross-section,so that a desired pattern can be created in a target portion of thesubstrate. The terms “light valve” and “Spatial Light Modulator” (SLM)can also be used in this context. Examples of such patterning devicesare discussed below.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate. An array of diffractive optical micro electrical mechanicalsystem (MEMS) devices can also be used in a corresponding manner. Eachdiffractive optical MEMS device can include a plurality of reflectiveribbons that can be deformed relative to one another to form a gratingthat reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror arrayemploying a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means.

In both of the situations described here above, the array ofindividually controllable elements can comprise one or more programmablemirror arrays. More information on mirror arrays as here referred to canbe gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193,and PCT patent applications WO 98/38597 and WO 98/33096, which areincorporated herein by reference in their entireties.

A programmable LCD array can also be used. An example of such aconstruction is given in U.S. Pat. No. 5,229,872, which is incorporatedherein by reference in its entirety.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe array of individually controllable elements may differ substantiallyfrom the pattern eventually transferred to a layer of or on thesubstrate. Similarly, the pattern eventually generated on the substratemay not correspond to the pattern formed at any one instant on the arrayof individually controllable elements. This may be the case in anarrangement in which the eventual pattern formed on each part of thesubstrate is built up over a given period of time or a given number ofexposures during which the pattern on the array of individuallycontrollable elements and/or the relative position of the substratechanges.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as, for example, the manufacture of DNA chips,MEMS, MOEMS, integrated optical systems, guidance and detection patternsfor magnetic domain memories, flat panel displays, thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist) or a metrology or inspection tool.Where applicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection systems, includingrefractive optical systems, reflective optical systems, and catadioptricoptical systems, as appropriate, for example, for the exposure radiationbeing used, or for other factors such as the use of an immersion fluidor the use of a vacuum. Any use of the term “lens” herein may beconsidered as synonymous with the more general term “projection system.”

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the beam of radiation,and such components may also be referred to below, collectively orsingularly, as a “lens.”

The lithographic apparatus may be of a type having two (e.g., dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.,water), so as to fill a space between the final element of theprojection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

Further, the apparatus may be provided with a fluid processing cell toallow interactions between a fluid and irradiated parts of the substrate(e.g., to selectively attach chemicals to the substrate or toselectively modify the surface structure of the substrate).

Lithographic Projection Apparatus

FIG. 1 schematically depicts a lithographic projection apparatus 100according to an embodiment of the invention. Apparatus 100 includes atleast a radiation system 102, an array of individually controllableelements 104, an object table 106 (e.g., a substrate table), and aprojection system (“lens”) 108.

Radiation system 102 can be used for supplying a beam 110 of radiation(e.g., UV radiation), which in this particular case also comprises aradiation source 112.

An array of individually controllable elements 104 (e.g., a programmablemirror array) can be used for applying a pattern to beam 110. Ingeneral, the position of the array of individually controllable elements104 can be fixed relative to projection system 108. However, in analternative arrangement, an array of individually controllable elements104 may be connected to a positioning device (not shown) for accuratelypositioning it with respect to projection system 108. As here depicted,individually controllable elements 104 are of a reflective type (e.g.,have a reflective array of individually controllable elements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device 116 for accurately positioning substrate 114with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF2 lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 may project animage of the array of individually controllable elements 104 ontosubstrate 114. Alternatively, projection system 108 may project imagesof secondary sources for which the elements of the array of individuallycontrollable elements 104 act as shutters. Projection system 108 mayalso comprise a micro lens array (MLA) to form the secondary sources andto project microspots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander 126, for example. Illuminator 124 may comprise anadjusting device 128 for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in beam 122. In addition, illuminator 124 willgenerally include various other components, such as an integrator 130and a condenser 132. In this way, beam 110 impinging on the array ofindividually controllable elements 104 has a desired uniformity andintensity distribution in its cross section.

It should be noted, with regard to FIG. 1, that source 112 may be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 may also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts the array of individually controllableelements 104 after being directing using beam splitter 118. Having beenreflected by the array of individually controllable elements 104, beam110 passes through projection system 108, which focuses beam 110 onto atarget portion 120 of the substrate 114.

With the aid of positioning device 116 (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 106 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the array ofindividually controllable elements 104 can be used to accurately correctthe position of the array of individually controllable elements 104 withrespect to the path of beam 110, e.g., during a scan. In general,movement of object table 106 is realized with the aid of a long-strokemodule (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem may also be used to position the array of individuallycontrollable elements 104. It will be appreciated that beam 110 mayalternatively/additionally be moveable, while object table 106 and/orthe array of individually controllable elements 104 may have a fixedposition to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106may be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 may be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 may be used to project a patterned beam 110 for use in resistlesslithography.

The depicted apparatus 100 can be used in four preferred modes:

1. Step mode: the entire pattern on the array of individuallycontrollable elements 104 is projected in one go (i.e., a single“flash”) onto a target portion 120. Substrate table 106 is then moved inthe x and/or y directions to a different position for a different targetportion 120 to be irradiated by patterned beam 110.

2. Scan mode: essentially the same as step mode, except that a giventarget portion 120 is not exposed in a single “flash.” Instead, thearray of individually controllable elements 104 is movable in a givendirection (the so-called “scan direction”, e.g., the y direction) with aspeed v, so that patterned beam 110 is caused to scan over the array ofindividually controllable elements 104. Concurrently, substrate table106 is simultaneously moved in the same or opposite direction at a speedV=Mv, in which M is the magnification of projection system 108. In thismanner, a relatively large target portion 120 can be exposed, withouthaving to compromise on resolution.

3. Pulse mode: the array of individually controllable elements 104 iskept essentially stationary and the entire pattern is projected onto atarget portion 120 of substrate 114 using pulsed radiation system 102.Substrate table 106 is moved with an essentially constant speed suchthat patterned beam 110 is caused to scan a line across substrate 106.The pattern on the array of individually controllable elements 104 isupdated as required between pulses of radiation system 102 and thepulses are timed such that successive target portions 120 are exposed atthe required locations on substrate 114. Consequently, patterned beam110 can scan across substrate 114 to expose the complete pattern for astrip of substrate 114. The process is repeated until complete substrate114 has been exposed line by line.

4. Continuous scan mode: essentially the same as pulse mode except thata substantially constant radiation system 102 is used and the pattern onthe array of individually controllable elements 104 is updated aspatterned beam 110 scans across substrate 114 and exposes it.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Exemplary

FIG. 2 is a simplified illustration of an optical projection systemincorporating a microlens array, according to one embodiment of thepresent invention. FIG. 2 illustrates in schematic form a particularlithographic apparatus of the type described in general terms in FIG. 1.The apparatus shown in FIG. 2 comprises a contrast device 1 having a twodimensional array of elements 2 on a lower surface thereof. An angularposition of each of the two dimensional array of elements 2 can beselectively controlled. A beam splitter 3 is positioned beneath contrastdevice 1. An illumination source 4 directs a beam of radiation 5 towardsbeam splitter 3, which reflects beam 5 onto the lower surface ofcontrast device 1.

One of the elements 2 of contrast device 1 reflects a component part ofbeam 5 back through beam splitter 3 and through projection opticsdefined by lenses 6, 7, and 8 onto substrate 10. In one example, alowermost lens 8 is a field lens that produces a substantiallytelecentric beam, which is directed towards a microlens array 9.Microlens array 9 comprises a two dimensional array of small lenses,each of which is arranged so as to focus light incident upon it onto anupper surface of substrate 10. Thus, for each of contrast elements 2 incontrast device 1 that acts as a mirror reflecting light to array 9, arespective one of the lenses in array 9 is illuminated and a respectivespot of light is projected by that lens in array 9 onto the uppersurface of substrate 10. In this arrangement, contrast device 1 isimaged on substrate 10.

FIG. 3 is a simplified illustration of components of the system shown inFIG. 2 and includes a displaceable substrate table, according to oneembodiment of the present invention. In this example, substrate 10 isshown supported on a substrate table 11 beneath microlens array 9. Theprojection optics are represented by a simple rectangle 12. Threecontrast elements 2 of contrast device 1 of FIG. 2 are shown aboveprojection optics 12. In this example, substrate table 11 is moved in alinear manner in a direction of arrow 13 beneath microlens array 9.

FIG. 4 is a schematic representation of the orientation of spots oflight projected onto a substrate in the system illustrated in FIG. 3,according to one embodiment of the present invention. This figure isillustrative of a relationship between a disposition of the individuallenses in microlens array 9 of FIGS. 2 and 3 and a direction ofdisplacement of substrate table 11 of FIG. 3. The direction ofdisplacement is represented in FIG. 4 by arrow 13. That direction isparallel to a line 14 that is inclined with respect to a line 15. Line15 extends parallel to a row of the lenses in microlens array 9. Eachlens projects light onto a different one of a rectangular array of spots16. The lenses are arranged in a two dimensional array that is slightlyinclined with respect to direction 13 of substrate table movement, suchthat the entire surface of substrate 10 can be exposed by appropriatecontrol of the illumination beams delivered to the respective lenses bythe respective elements 2 of contrast device 1. Each lens can in effect“write” a continuous line on the surface of substrate 10 and, given thedisposition of the lenses relative to the direction of substratemovement, those lines are sufficiently close together to overlap. Inorder to expose a selected two dimensional area of substrate 10,substrate 10 is advanced beneath microlens array 9. The individuallenses beneath which the area to be exposed is positioned at any onetime are illuminated by appropriate control of the associated elements 2of contrast device 1.

FIG. 5 is a schematic representation of a contrast device and contrastaperture in a first disposition of the contrast device, according to oneembodiment of the present invention. FIG. 6 corresponds to FIG. 5 aftera change in the disposition of the contrast device, according to oneembodiment of the present invention.

In one example, it is desirable to be able to control the intensity ofradiation reaching each of spots 16 on substrate 10. In this example,the position of individual reflective elements 2 of the contrast device1 are adjusted, for example as shown in FIGS. 2 and 3. This can be doneso that only part of a beam of light reflected by an individualreflective element 2 reaches the associated lens in microlens array 9.This is illustrated schematically in FIGS. 5 and 6.

FIG. 5 shows a reflective element 2 of a contrast device 1 directing abeam of radiation along an axis 17 directed at a center of a circularaperture in a contrast aperture plate 18.

FIG. 6 shows element 2 after it has been tilted relative to its positionin FIG. 5, so that the beam is partially offset relative to contrastaperture plate 18. In the arrangement of FIGS. 5 and 6, contrastaperture plate 18 defines a pupil that is imaged at substrate 10.

In one example, as illustrated in FIG. 5, the intensity of the radiationis symmetrical about a center line 19 passing through the center ofcontrast aperture plate 18. This intensity is represented by a curve 20.Thus, a beam passing through contrast aperture plate 18 that reaches alens 21 of microlens array 9 is projected onto substrate 10 to form anillumination spot on substrate 10 that is symmetrical about a centralaxis of lens 21, which axis is represented by line 22.

In one example, as illustrated in FIG. 6, the intensity distributionrepresented by line 20 is offset relative to center line 19 of contrastaperture plate 18, so too is the distribution of light on substrate 10beneath lens 21. As a result, there is an undesirable coupling betweenspot position and intensity on substrate 10. Effectively, a portion ofthe spot projected onto substrate 10 to the left of line 22 in FIG. 6receives radiation of a greater intensity than a portion of the spot tothe right of line 22. Thus, although displacing element 2 of contrastdevice 1 from the position shown in FIG. 5 to the position shown in FIG.6 does reduce the intensity of the radiation reaching substrate 10, thesymmetrical distribution of that radiation that is achieved in the caseillustrated in FIG. 5 is not achieved in the case illustrated in FIG. 6.

FIGS. 7 and 8 illustrate the distribution of a radiation beam relativeto contrast aperture plate 18 given the dispositions of contrast device1 shown in FIGS. 5 and 6, according to embodiments of the presentinvention.

FIG. 7 illustrates, through line 23, a periphery of the aperture in thecontrast aperture plate 18 of FIG. 5 and, through line 24, the positionrelative to that aperture of the beam of radiation.

In contrast, FIG. 8 shows, through line 28, the relative disposition ofperiphery 28 of the aperture and of beam of radiation 24 in the caseillustrated in FIG. 6. Beam 24 overlaps aperture 28 only in a shadedarea, which is clearly asymmetrical.

FIG. 9 shows an arrangement in which two contrast devices are adjustedto produce a radiation distribution that is symmetrical relative to acontrast aperture, according to one embodiment of the present invention.Symmetry is achieved by relying upon two contrast devices 2 toilluminate a same lens 21 of microlens array 9. The distribution of theradiation reflected by the left hand element 2 in FIG. 9 is indicated byline 25 in FIG. 9, whereas the intensity distribution related to theright hand element 2 is indicated by line 26. These two distributionsare symmetrical relative to a plane through center line 19 of theaperture in aperture plate 18. This symmetry is maintained throughoutthe projection system, such that there is symmetry about a planeperpendicular to FIG. 9 and through center line 22 of lens 21.

FIG. 10 illustrates a distribution of radiation given the dispositionsof the two contrast devices shown in FIG. 9, according to one embodimentof the present invention. Line 23 is the periphery of the aperture inaperture plate 18 and lines 27 and 28 are the periphery of the beamsprojected by respective elements 2 of contrast device 1 shown in FIG. 9.Symmetry is maintained about a plane perpendicular to FIG. 9 indicatedby line 29, which passes through line 22 in FIG. 9.

In the example illustrated in FIGS. 9 and 10, a group of two ofreflective elements 2 reflect light towards the same single lens ofmicrolens array 9. The two reflective elements 2 making up the group arecontrolled, such that they rotate in opposite directions. Thus, theydeflect the associated beams of radiation in directions that areinclined to each other by 180°.

It is to be appreciated that a group could comprise more than tworeflective elements 2 and still maintain the required symmetry. Forexample, three reflective elements 2 could be arranged in a groupassociated with one lens of microlens array 9.

FIGS. 11 and 12 illustrate distribution of radiation in an arrangementwith three symmetrically disposed contrast devices and foursymmetrically disposed contrast devices, respectively, according tovarious embodiments of the present invention.

As schematically represented in FIG. 11, three reflective elements arearranged as a group to progressively deflect radiation in directionsinclined at 120° intervals with respect to each other. The outline ofthe aperture in the aperture plate 18 is again represented by line 23,whereas the three separate beams deflected by the group of threeassociated reflective elements are represented by lines 30.

As schematically represented in FIG. 12, four individually controllableelements are arranged as a group to progressively deflect radiation indirections inclined at 90° intervals with respect to each other. Theoutline of the aperture in the aperture plate 18 is again represented byline 23, whereas the four beams reflected by the group of fourassociated reflective elements are indicated by lines 31.

In each of the cases illustrated in FIGS. 10, 11 and 12, symmetry ismaintained relative to planes indicated by straight lines.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A lithographic apparatus, comprising: an illumination systemconfigured to condition a beam of radiation; an array of individuallycontrollable elements configured to pattern the beam; and a projectionsystem configured to project the patterned beam onto a target portion ofa substrate, the projection system defining a pupil and comprising anarray of lenses, wherein the array of lenses is arranged to direct arespective part of the patterned beam towards a respective part of thetarget portion of the substrate, wherein the individually controllableelements are arranged in groups, such that the patterned beam isdirected by each element in each respective group towards acorresponding lens in the lens array, and wherein the individuallycontrollable elements in each group are controlled together to directthe patterned beam away from the pupil, whereby a pattern imparted ontothe beam by that group of individually controllable elements issubstantially symmetrical with respect to the pupil.
 2. The apparatus ofclaim 1, wherein the individual elements in each of the groups areconfigured to direct the patterned beam in different directions awayfrom the pupil.
 3. The apparatus of claim 1, wherein the individuallycontrollable elements are selectively controllable to progressivelydirect a respective part of the patterned beam away from the pupil,whereby an amount of the patterned beam passing through the pupil isprogressively varied.
 4. The apparatus of claim 1, wherein each of thegroups of the individually controllable elements comprises two elementsthat progressively direct the patterned beam away from the pupil indirections inclined by 180° with respect to each other.
 5. The apparatusof claim 1, wherein the groups of the individually controllable elementscomprises three elements that progressively direct the patterned beamaway from the pupil in directions inclined at 120° intervals withrespect to each other.
 6. The apparatus of claim 1, wherein the groupsof the individually controllable elements comprises four elements thatprogressively direct radiation away from the pupil in directionsinclined at 90° intervals with respect to each other.
 7. The apparatusof claim 1, wherein the individually controllable elements are mirrorsthat are tilted progressively away from a position in which the beamreflected by that mirror is symmetrical with respect to the pupil. 8.The apparatus of claim 1, further comprising: a beam splitter configuredto reflect the beam from the illumination system toward the individualcontrollable elements and to transmit the patterned beam from theindividually controllable elements towards the array of lenses.
 9. Theapparatus of claim 1, wherein the pupil is defined by a projection lenscontrast aperture plate.
 10. A device manufacturing method, comprising:(a) controlling individually controllable elements in an array ofindividually controllable element in groups to pattern a beam ofradiation; (b) selectively controlling the individually controllableelements to direct a respective part of the patterned beam away from apupil in a projection system; (c) using the individually controllableelements in each of the groups to direct the patterned beam towards asame lens in an array of lenses in a projection system; (d) projectingthe patterned beam onto a target portion of a substrate using the arrayof lenses in the projection system; and (e) using the individuallycontrollable elements to direct the patterned beam of radiation awayfrom the pupil, whereby the pattern imparted to the beam by each of thegroups is substantially symmetrical with respect to the pupil.
 11. Themethod of claim 10, wherein step (e) directs the patterned beam indifferent directions away from the pupil.
 12. The method of claim 11,wherein step (e) progressively directs the pattered beam in thedifferent directions.
 13. The method of claim 10, wherein step (d)wherein an amount of the patterned beam passing through the pupil isprogressively varied.
 14. The method of claim 10, further comprising:forming one of the groups with two of the individually controllableelements, which are progressively adjusted to direct the patterned beamaway from the pupil in directions inclined by 180° with respect to eachother.
 15. The method of claim 10, further comprising: forming one ofthe groups with three of the individually controllable elements, whichare progressively adjusted to direct the patterned beam away from thepupil in directions inclined by 120° with respect to each other.
 16. Themethod of claim 10, further comprising: forming one of the groups withfour of the individually controllable elements, which are progressivelyadjusted to direct the patterned beam away from the pupil in directionsinclined by 90° with respect to each other.
 17. The method of claim 10,further comprising: using mirrors as the individually controllableelements, wherein the mirrors are tilted away from a position in whichthe patterned beam deflected by each of the mirrors is symmetrical withrespect to the pupil.
 18. The method of claim 10, further comprising:directing the beam towards the individual controllable elements using abeam splitter; and directing the patterned beam from the individuallycontrollable elements towards the array of lenses via the beam splitter.19. The method of claim 10, wherein a projection lens contrast apertureis positioned to define the pupil.